With continuous development of aerospace technologies and increase of national defense construction requirements, more attention is paid to diagnosis and performance evaluation of an engine combustion flow field. A highly efficient combustion flow field measurement means can provide important references for improving combustion efficiency of an engine. In conventional combustion flow field measurement, a contact measurement means, for example, a pneumatic velocity probe, a gas sampling probe, or a thermocouple temperature probe, is mainly used. Contact measurement has disadvantages such as a high maintenance cost, a high failure rate, a low response speed, and inconvenience for carrying and mounting. In addition, an intrusive probe may destruct a measured flow field, and generate a shock wave, causing severe interference to an air flow and affecting measurement accuracy. Therefore, it is necessary to develop an advanced non-contact measurement means.
The tunable diode laser absorption spectroscopy (TDLAS) is an online measurement technology, and has advantages such as high sensitivity, a high anti-noise capability, and a high environmental adaptability. TDLAS has been extensively applied to diagnosis and research in the combustion flow field and a propulsive flow field in recent years. TDLAS is a line-of-sight measurement technology, and only an average value in a lightpath direction can be obtained. However, in an actual flow field, due to effects of flow mixing, a phase change, a chemical reaction, a heat exchange with wall surfaces, etc., there is an obvious gradient change in a light propagation direction. An average value of a gas parameter in a single path cannot meet a requirement for predicting gas flow characteristics. Therefore, information about spatial distribution of light on a same plane needs to be added to meet a requirement for obtaining two-dimensional distribution of gas. However, because measurement space in the combustion flow field is limited, a higher requirement is imposed on a structural design of a measurement apparatus. In addition, because an experimental apparatus needs to acquire data in dozens of lightpaths, and an amount of data in an experiment is huge, high requirements are imposed on a structure of a measurement apparatus, a measurement method, data transmission, and data processing.
Facing a challenge of two-dimensional reconstruction and measurement of gas parameters in the combustion flow field, persons skilled in the art are greatly concerned about technical issues on how to design the measurement apparatus, improve the measurement method, reduce measurement time, improve measurement accuracy, and implement two-dimensional measurement of gas temperature and component concentration in the combustion flow field.
The following documents and reports relate to designs of a method and an experimental apparatus for two-dimensional reconstruction and measurement of gas parameters in a combustion flow field.
1. “Tunable diode laser absorption technique development for determination of spatially resolved water concentration and temperature” (48th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, AIAA-2010-0299), a dissertation by E Bryner, etc. University of Virginia, U.S.A. A filtered back projection method is used to measure two-dimensional distribution of temperature and component concentration at an outlet of a combustion chamber. Because a complete projection of a measured area is required, during an experiment, data in a total of 1800 lightpaths is acquired from 72 angles in a motion and rotation mode, and data acquisition in the experiment takes nearly one hour.
2. “Application of Diode-Laser-Based Measurements in Hypersonic Flows” (50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, AIAA-2012-0555), a dissertation by Michael S. Bown, Air Force Research Laboratory, U.S.A. With respect to distribution of temperature and H2O concentration at an outlet of a HIFiRE-2 combustion chamber, a ground experiment is carried out. At the outlet, six laser lightpaths are installed in a horizontal direction, and eight laser lightpaths are installed in a vertical direction. Two-dimensional distribution of temperature and H2O concentration at the outlet of the combustion chamber is measured. Because there are projections in two directions only, an obtained result can be used only for qualitative analysis.
3. “Tunable diode laser absorption spectroscopy-based tomography system for on-line monitoring of two-dimensional distributions of temperature and H2O mole fraction” (Review of Scientific Instruments, issue 1, volume 87, 2016), a dissertation by Xu Lijun, etc., Beihang University. A sector beam projection mode is used, five projection angles are selected, and a cylindrical prism is used for beam splitting. A total of 60 beams are obtained, and a quantity of laser transmitters is reduced effectively. In an experimental apparatus, a cylindrical lens, an anamorphic prism, etc. all need to be fixed on a lab table, and large space is occupied. The experimental apparatus is applicable only to a lab desktop experiment and research. It is difficult to use the experimental apparatus to measure an engine combustion flow field.
4. “Parallel-Beam Tomography Based on TDLAS” (issue 1, volume 46, 2014), a dissertation by Li Fei, etc., Institute of Mechanics, Chinese Academy of Sciences. A rotary measurement apparatus based on six parallel beams is designed. Due to a limitation of hardware conditions in an experiment, a rotation angle is 90°, and therefore, a great error exists in a reconstruction result.